Hatching blastocyst embryo. Coloured scanning electron micrograph (SEM) of a human embryo at the blastocyst stage, five days after fertilisation. It is seen at right hatching from a hole in the zona pellucida (at left), a protein shell that originally surrounded the unfertilised egg. The blastocyst is a hollow ball of cells with a fluid centre. Each cell is called a blastomere. Most of these embryo cells will form the placenta and membranes around the embryo, and only a small group (the inner mass) form the embryo proper. At this stage the blastocyst has moved into the uterus (womb) and is preparing to implant on the womb wall. Magnification: x300 at 6x7cm size. Mag;x290 at original 6x7cm size.